of the type shown in Fig. 18.4. On stretching the specimen, the force required to propagate a tear from the notch tip can be determined. This type of determination, which is commonly used in the rubber industry, reveals that there is little difference between the different material types.

Decontamination of elastomeric impression materials is required in order to prevent crossinfection. It is a requirement of the ISO Standard that the manufacturer provides details of a method which is suitable for any particular product. Typically, the method of decontamination involves rinsing followed by a short soak in a solution of hypochlorite or glutaraldehyde. Such treatment is unlikely to harm the silicone or polysulphide materials. However, the polyether materials are hydrophilic and great care must be taken when soaking impressions in aqueous solutions – lest the material absorbs water and swells.

Jaw Registration: Increasingly elastomeric materials (both polyether and addition cured silicone rubber) are being used to record the relationship between the upper and lower jaws. The materials are usually in a gun-mix format and of sufficient viscosity that they will not drip off the teeth when applied from the applicator. One clinical problem

with their use is that the materials will record too much surface detail from the occlusal areas of the molar teeth which is not recorded and/or cannot be re-engaged by the registration when placed onto the working model. This results in folds of registration material being trapped against the surface of the casts making the whole process less accurate.

Table 19.7 gives a qualitative summary of the comparative properties of the materials.

19.7 Suggested further reading

Craig, R.G., Urquiola, N.J. & Liu, C.C. (1990) Comparison of commercial elastomeric impression materials. Oper. Dent. 15, 94.

Donovan, T.E. & Chee, W.W. (2004) A review of contemporary impression materials and techniques. Dent. Clin. North Am. 48, 445.

ISO 4823 Dental Elastomeric Impression Materials. Wassell, R.W. & Abuasi, H.A. (1992) Laboratory

assessment of impression accuracy by clinical simulation. J. Dent. 20, 108.

Wassell, R.W., Barker, D. & Walls, A.W.G. (2002) Impression materials and technique. In A Clinical Guide to Crowns and other Extra-coronal Restorations (R.W. Wassell, A.W.G. Walls, J.G. Steele & F. S. Nohl, eds), pp. 9–17. BDJ Books: London.

20.1 Introduction

Direct filling materials are used for chairside restoration of teeth. They differ from indirect restorations, such as crowns, bridges or inlays, because no laboratory stage is involved in the provision of the restoration.

Teeth may need restoring for a variety of reasons. Destruction of tooth substance caused by dental caries may result in the loss of considerable quantities of enamel and dentine. Trauma may cause fracture and loss of parts of teeth. In this case the anterior teeth are most vulnerable and those teeth affected may be otherwise sound and caries-free. A third factor causing loss of tooth substance is wear. This often arises due to overzealous brushing using an abrasive dentifrice but may also arise due to a peculiarity of the diet, working environment or habits of the patient. High frequency of exposure of teeth to acids in food and drink or from regurgitated gastric juice is of growing concern in relation to the wear of teeth involved.

The parts of teeth which require replacement by a restorative material vary in size, shape and location in the mouth. Thus, at one extreme, it may be necessary to restore a large cavity which extends over the mesial, occlusal and distal surfaces of a molar tooth. An entirely different situation is the restoration of the corner of an incisor which has been lost in an accident. The requirements of materials used in these and other applications vary and it is not surprising that no single restorative material is suitable for all cases. For some situations the strength and abrasion resistance of the material may be the prime consideration. In other situations appearance and adhesive properties may become more important.

The factor which is generally used to assess the success or failure of a restorative material for any

application is durability. In this context the term refers to the life expectancy of the restoration and the life expectancy of the surrounding tooth substance and how it may be affected by the presence of the restoration. Durability depends on the physical and biological properties of the restorative material.

The acceptance of the material by the profession also depends on the ease with which it can be handled in the surgery.

20.2 Appearance

Logic dictates that materials used as restorations in ‘visible’ cavities in anterior teeth will be subjected to greater scrutiny of their appearance than materials used in occlusal cavities in posterior teeth. However, patients’ awareness of the variety of material options available to the dentist may result in requests for the use of tooth coloured materials even where appearance may seem to be of minor significance to the trained professional eye.

20.3 Rheological properties and setting characteristics

Many restorative materials are supplied as two or more components which require mixing. Thorough mixing should be easy to accomplish in a reasonable time. After mixing, the ease of handling depends on factors such as viscosity, tackiness and setting characteristics such as working time and setting time. Different techniques must often be adopted to handle different materials. Whereas some materials readily flow into the prepared cavity under little pressure some products require ‘packing’ under considerable pressure. When materials remain tacky for some time after

mixing they may be difficult to handle because they adhere to instruments. Working time should be sufficiently long to enable manipulation and placement of materials before the setting reaction reaches the stage at which continued manipulation is either difficult or would adversely affect the structure and properties of the final set material. Setting times should, ideally, be short for the comfort and convenience of both the patient and clinician.

20.4 Chemical properties

Filling materials are required to withstand the hostile environment of the oral cavity for many years without dissolving, degrading or eroding. Thus, the materials must withstand large variations in pH and a variety of solvents which may be taken into the mouth in drinks, foodstuffs and medicaments. In addition, metallic filling materials should not undergo excessive corrosion or be involved in the development of electrical currents which may cause galvanic pain.

20.5 Thermal properties

Filling materials should, ideally, be good thermal insulators, protecting the dental pulp from the harmful effects of hot and cold stimuli. The thermal insulating properties of filling materials are best characterised in terms of thermal diffusivity (p. 21) since this describes the behaviour of materials subjected to transient thermal stimuli. Hence, the value of thermal diffusivity should ideally be low. Materials having relatively high values of thermal diffusivity may require the use of an insulating cavity base material.

The thermal expansion and contraction of a filling which occurs, for example, when a patient takes a hot or cold drink, should match that of the surrounding tooth substance. Thus, materials should have values of coefficient of thermal expansion (p. 23) similar to those of enamel and dentine. A large mismatch of values may result in leakage of fluids down the margin between the filling material and surrounding tooth. However, the extent of dimensional changes caused by thermal fluctuations may be minimized by the transient nature of most intra-oral thermal stimuli and the relatively low thermal diffusivity of most nonmetallic restorative materials. A transient temperature change of 10ºC applied for only a second to

the surface of a polymeric material is likely to cause an insignificant change in dimensions of anything other than the surface layer of material directly exposed to the stimulus.

20.6 Mechanical properties

The mechanical property requirements of filling materials vary considerably depending on the type of tooth and the particular surface being restored. For the restoration of large cavities, involving two or more surfaces of a posterior tooth, a strong material with adequate abrasion resistance is required to withstand the large stresses developed in that region of the mouth. When materials are subjected to direct masticatory loading they should also be able to resist plastic deformation or creep. For a small occlusal cavity in a posterior tooth the properties of the material may not be as critical since it is totally supported by enamel. For a small interproximal cavity in the anterior region the major factor for consideration may be abrasion resistance. That surface of the tooth is not involved in direct contact with other teeth but may be subjected to considerable toothbrush/dentifrice abuse.

It is recognized that the marginal seal between filling material and tooth substance may be destroyed if the material is able to undergo elastic deformation under loading. A high value of modulus of elasticity is therefore beneficial.

20.7 Adhesion

It is recognized that an adhesive bond between restorative material and tooth substance is desirable though not always attainable. Such a bond effectively seals the margin, preventing the ingress of fluids and bacteria. In addition, the adhesive bond potentially reduces the amount of cavity preparation required in order to achieve retention of the filling. The subject of adhesion and adhesive materials is dealt with in Chapter 23.

20.8 Biological properties

Filling materials, in common with all other dental materials, should be harmless to both the operators and patients. The specific requirements of these products relate to their effect on the dental pulp. They should not, either directly or indirectly

cause irritation to the pulp, nor should they contain substances which are able to leach out and cause irritation. It should be remembered that dentine contains many tubules which are capable of transporting chemicals from the base of fillings to the pulp. Biologically bland cavity bases or cavity linings are often used when filling materials are not sufficiently bland to be used directly.

20.9 Historical

Directly placed fillings offer the advantage of potential savings in time and cost coupled with greater convenience when compared with restorations which are constructed by an indirect process. Historically, the three earliest materials to gain acceptance as direct fillings were cohesive gold foil, dental amalgam and silicate cements. Of these, only amalgam is still widely used and its continued use is subject to considerable scrutiny as concerns over the use of heavy metals, and particularly mercury, increase. Cohesive gold is discussed in Section 7.2. Although it has some attributes its use is now very limited due to the cost of the material and the involved technique required.

Silicate cements were the first directly placed tooth coloured filling materials. They are now rarely, if ever, used but some important lessons can be learned from a consideration of their composition and properties. The materials were supplied as a powder (alumino silicate glass) and liquid (aqueous phosphoric acid) which were mixed together to bring about setting. Setting was through a complex series of acid–base reactions. One of the most crucial aspects of the manipulation of the cement was the need to protect the material from moisture during setting. Contamination could lead to a marked increase in solubility – a fact which also applies to some currently used cements which have similar setting reactions to that of the silicates. Dentists using silicates were aware of the highly acidic nature of the cements and a lining was considered essential to protect the dentine and pulp from the phosphoric acid component. Pulpal problems were invariably

attributed to the harmful effects of acid and this is of interest in respect of the modern trend to utilize acid treatments of dentine for conditioning prior to the placement of fillings. There has recently been a lot of work carried out to review the way in which the pulp reacts to chemical attack. It is now thought that many of the pulpal problems associated with silicates had nothing to do with traumatization by acid but were more likely related to the fact that silicates offer no means of adhesion to the tooth and therefore cannot prevent microleakage from occurring.

Much of the evidence (some anecdotal) on the beneficial effects of fluoride release from fillings comes from experience of the use of silicates. The materials were able to release fluoride over a long time and the incidence of decay around silicate fillings was always low.

The mechanical properties of silicates were far from ideal. The materials were relatively weak and brittle compared with other materials and this limited their use to low-stress situations – particularly class III cavities. Experience with silicates clearly illustrated the importance of using the correct powder–liquid ratio for cements and pointed the way to the improvements which could be obtained with encapsulation. Silicate materials offered a very good example of the way in which material durability can depend on the oral hygiene and dietary habits of the patient. The solubility and erosion of the cements was very pH dependent. A well constructed restoration in a patient practising good oral hygiene and having a low acid content diet could survive 20 years or more. On the other hand, an acidic environment arising from poor oral hygiene and/or a habitual acid drink intake caused rapid erosion of the filling. To some extent, the same is true of some modern day restorative cements. The use of silicates declined markedly with the advent of resin based products

– particularly composites in the 1960s. Glassionomer cements, which have been used since the 1970s, bear some similarity to the silicates. They set by a similar acid–base reaction and have a glass component which is similar to that of the silicates.